In the past forty years or so, sizing agents have been sought that can be used in the neutral or alkaline pH range. Problems with paper making in the acid range where traditional rosin sizing agents have been used are well know, corrosion to the paper machine and browning and embrittlement of paper with age. Synthetic sizing agents alkenyl succinic anhydride (U.S. Pat. No. 3,102,064) and alkyl ketene dimer (U.S. Pat. No. 3,130,118) were developed for use under neutral and alkaline conditions. Rosin products are claimed to be useful at neutral or near neutral pH levels if they are made into cationic dispersions or if they employ modified rosins. Thus, U.S. Pat. No. 3,966,654 discusses cationic rosin emulsions, a specific example of which contains a water-soluble cationic aminopolyamide-epicholorhydrin resin, that were applied to pH 6.5. U.S. Pat. No. 4,943,608 discloses rosin emulsion sizing agents comprising fortified rosin, an at least partially quarternized product copolymer principally consisting of a (meth)acrylic acid alkylaminoalkyl ester or amide monomer and water. These products were tested to pH 6.8 in hand sheet sizing evaluations. U.S. Pat. No. 5,438,087 discloses rosin emulsions containing cationic acrylamide and/or methacrylamide polymers having hydrophobic groups. These products were tested at pH 7 with bleached kraft pulps and at unknown pH values with other pulps, used with retention aids.
Modifications to the rosin are disclosed in three U.S. patents. U.S. Pat. No. 4,540,635 discloses rosins that have been esterified with tertiary amino alcohol and also reinforced with formaldehyde and/or .alpha.,.beta.-unsaturated carbonyl that are useful for neutral paper making. U.S. Pat. No. 4,842,691 discloses rosins reacted with at least one polyhydric alcohol selected from among a trihydric alcohol and a tetrahydric alcohol both consisting of carbon, hydrogen and oxygen, and useful at a pH of about 6 to about 9. U.S. Pat. No. 5,399,660 discloses rosins modified by being a diester and having a dicarboxylic acid or acid anhydride group or being modified with polyhydric alcohols, polybasic (tribasic or more) carboxylic acid or its anhydride and .alpha.,.beta.-unsaturated polybasic acid or variants of these useful as neutral paper sizing agents.